1. Field of the Invention
The invention relates to a blue light emitting diode, and more particularly a blue light emitting diode, which has such an electrode structure as to prevent the concentration of current density, causative of a rapid temperature increase in the diode, thereby increasing resistance to electrostatic discharge (ESD) and lowering a deriving voltage without significantly altering the laminate structure of the electrode.
2. Description of the Related Art
Recently, a light emitting diode which is able to emit light in the region of short wavelength (ultraviolet light to green), particularly blue light has gained public popularity. Such semiconductor materials include ZnSe (II-VI), nitrides such as GaN, InN, AlN (III-V) and nitride mixtures combining these nitrides in a certain ratio and particularly GaN are widely used.
Growing of GaN crystal is effected by MOCVD (Metal Organic Chemical Vapor Deposition) method. The MOCVD method is carried out by supplying reactive gas of an organic compound into a reaction chamber at a temperature of 700 to 1200xc2x0 C. to grow crystals in an epitaxial layer on a substrate, in which sapphire or SiC is used as the substrate. The reason that sapphire or SiC substrate is used is that there exist no substrates commercially available that can achieve lattice matching with the nitride crystal while having the same crystalline structure as the nitride crystal. Also, it can hardly be expected that the growth process of an epitaxial layer on such substrate would form a good quality of crystal, due to the stress resulting from lattice mis-matching. Therefore, a buffer layer is used as a low temperature-growing layer between the substrate and epitaxial layer.
Also, the blue light emitting diode with such a limited structure differs from the general light emitting diode in terms of the driving method, as explained below with reference to FIGS. 1a and 1b. 
FIG. 1 schematically shows the difference in the driving manner between a general light emitting diode (for example, a class of LEDs using GaAs, GaP, etc) and a blue light emitting diode (III-nitride). In the general light emitting diode, as shown in FIG. 1a, a light emitting diode chip is operated using a total structure including a wafer which acts as a substrate for crystal growth. However, as shown in FIG. 1b, the blue light emitting diode is operated through a thin structure fabricated on a chip, but not through the substrate. That is, the blue light emitting diode has a planar type structure using the insulating substrate as a sapphire, unlike the general light emitting diode.
Further, the blue light emitting diode is known to need a relatively high driving voltage at a constant current, compared to the general light emitting diode. FIGS. 2a and 2b show the voltage-current characteristics of an infrared light emitting diode (A), a red light (wavelength 635 nm) emitting diode (B) and a blue light (wavelength 450 nm) emitting diode (C) in a driving region of a forward direction. The blue light emitting diode requires a driving voltage of about two times as high as the red light emitting diode at a rated current of 20 mA. It is thought that such a high driving voltage is attributed to properties of GaN semiconductor layer and the planar type structure.
As described above, the blue light emitting diode suffers from problems in two aspects. First, the blue light emitting diode must adopt a deriving method for use in a planar type structure owing to the structural limit of growing a semiconductor layer on a sapphire substrate and a buffer layer so as to grow crystals with prevention of lattice mismatching. Another problem with the blue light emitting diode is the inherent feature of requiring a higher driving voltage as compared to general light emitting diodes. Consequently, the driving method and the high driving voltage of the blue light emitting diode may lead to reduced reliability and deteriorated quality of products.
FIG. 3a is a plan view of the conventional blue light emitting diode and FIG. 3b is a sectional view of the diode, taken along line Axe2x80x94A in FIG. 3a. Referring to FIGS. 3a and 3b, problems caused by the above-described restrictive problems of the blue light emitting diode will be explained in detail. As shown in FIG. 3a, the conventional blue light emitting diode include a sapphire substrate 1, a buffer layer 2 formed on the substrate 1, an n-type nitride semiconductor layer 3 comprising a central part R1 in a predetermined region and a peripheral part R2 surrounding the central part R1, and a laminated structure formed on the n-type nitride semiconductor layer 3.
The laminated structure has an active layer 4 made of intrinsic nitride semiconductor crystal in the central part R1 on the N-type nitride semiconductor 3, a p-type nitride semiconductor layer 5 formed on the active layer 4, a metal layer 6 atop the semiconductor crystal layer 5, and a first electrode 7, corresponding to a P electrode, formed in a predetermined region on the metal layer 6. Also, the light emitting diode includes a second electrode 8 as a N electrode formed in the peripheral part R2 over the N-type nitride semiconductor layer 3 while keeping a predetermined distance space from the central part R12 over the N-type nitride semiconductor layer 3.
In such conventional blue light emitting diode, current flows as injected carriers move on the surface of the diode and at the interface between the electrodes in the characteristic driving manner of the planar type structure. Also, the blue light emitting diode requires a high driving voltage across a given area for light emission, thereby forming a flow of a great quantity of injected carrier (herein electron). The current path Rp formed by the above flow of the carriers is distributed in accordance with the area of the electrode formed at an upper position. In FIG. 3a, therefore, the current density distribution is very high in the region Rd defined with a dotted line, and decreases gradually toward the periphery. The higher current density at the region Rd defined with the dotted line leads the temperature of the entire chip to increase, resulting in reducing the light output.
In consequence, since the conventional blue light emitting diode of the planar type structure requires a high driving voltage, defects existing in the region where a high current density is generated causes the chip temperature to be increased as well as incurring quality deterioration, for example, weak resistance to electrostatic discharge (ESD), which cause fundamental problems in achieving the reliability and quality stabilization of products.
Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a blue light emitting diode with an improved electrode structure which is capable of effectively dispersing the current density concentration causative of local temperature increase in the blue light emitting diode without requiring a significant structural change.
Another object of the present invention is to provide a blue light emitting diode which is highly resistant to ESD with a resulting improvement in terms of the quality and reliability of products.
Still another object of the present invention is to provide a blue light emitting diode in which the driving voltage is reduced and the rapid increasing of a temperature occurring locally in the chip is suppressed.
In order to achieve the above object, the present invention provides a blue light emitting diode comprising an insulating substrate, typically in a square shape, and a first conductive nitride semiconductor layer formed on the insulating substrate to have a surface divided into a central part and a peripheral part. The peripheral part is provided over the surface adjacent to and along the edges of the nitride semiconductor layer and the central part surrounded by the peripheral part.
Also, the blue light emitting diode includes a laminate structure formed over the central part of the nitride semiconductor layer, in which the laminate structure comprises a nitride active layer on the nitride semiconductor layer, a second conductive nitride semiconductor layer formed on the active layer, a transparent metal layer formed on the second conductive nitride semiconductor layer and a first electrode formed over a part of the transparent metal layer.
Further, the blue light emitting diode includes a second electrode formed over the peripheral part of the first conductive nitride semiconductor layer which is not covered by the laminate structure. As the insulating substrate, sapphire substrate can be used. It is also possible to use a sapphire substrate further comprising a GaN buffer layer formed thereon.
Here, the first electrode is referred to as a P electrode and the second electrode is referred to as an N electrode. The peripheral part on the first conductive nitride semiconductor layer (the layer expressed by Si-doped GaN) means the edge exposed with no active layer formed, that is, a surface part of the Si-doped GaN layer surrounding the active layer. In a predetermined region of this part, a second electrode is formed. Defined as a region in which the active layer is formed, the central part on the first conductive nitride semiconductor layer is a convex plane part surrounded by the peripheral part.
The blue light emitting diode may comprise the buffer layer made of GaN, the first conductive nitride semiconductor layer made of Si-doped GaN, the active layer made of In1-xGaxN (0 less than xxe2x89xa61) and the p-type nitride semiconductor layer made of All-xGaxN (0 less than xxe2x89xa61) and Mg-doped GaN. The above composition describes a blue light emitting diode that is currently, most commonly embodied. This is applicable to the blue light emitting diode according to another embodiment of the present invention.
In such a light emitting diode, the locations of the first electrode and the second electrode are changed and extensions of these electrodes are formed, whereby the current density can be effectively dispersed. As a result, the concentration of current density, causative of a rapid temperature increase, can be avoided only by the change of the location of the electrode and the formation of extensions thereof without significant alteration of the structure of the diode. In addition, it is possible to increase the resistance to ESD and reduce the driving voltage.
The reason why the present invention is interested in the improvement of the electrode structure is that the problems with conventional blue light emitting diode, including temperature increase attributable to the current density concentration, weak resistance to ESD, and high driving voltage, can be eliminated only by a metal patterning process with requiring no significant changes in other fabricating processes for the blue light emitting diode, nor employment of new equipments and materials.
The most important thing to be considered in the electrode structure is which type of electrode structure, that is, which type of locations and extensions of electrodes can disperse more effectively the current density. Leading to the present invention, the extensive and thorough research and experiments were conducted by the present inventors, in which the first electrode and the second electrode were set at various locations while the distance from the central part to the peripheral part was varied. As a result, there were found preferred embodiments which have an excellent capability of dispersing the current density and high resistance to ESD, and are operable at reduced driving voltage, thereby enhancing the quality and reliability.